1. Field of the Invention
This invention relates to an anamorphic prism typically adapted to compress or expand the incident light beam in a particular direction of cross section of the beam before letting it exit therefrom, to an optical head using such a prism and also to an optical signal recording/reproduction device using such an optical head.
2. Related Background Art
A variety of optical recording/reproduction devices have been developed in recent years and are currently very popular. However, optical disc devices for signal recording/reproduction are accompanied by the problem that the recording performance of the device can vary depending on the profile of the spot of light formed on the disc. Many optical disc devices are made to comprise an anamorphic prism that can change the magnification of the outgoing light beam relative to the incoming light beam in a particular direction of cross section thereof by compressing or expanding the incident light beam. Normally, a semiconductor laser is used as the light source of such a system and the divergence angle of the beam emitted from the semiconductor laser is about 10 degrees at full-width half maximum (FWHM) in a direction parallel to the pn junction plane (θ//direction) and about FWHM 20 to 30 degrees in a direction perpendicular to the pn junction plane (θ⊥ direction) (the ratio of the divergence angles, or θ⊥/θ// is referred to as aspect ratio). Therefore, without such a conversion of magnification of the emitted beam, the intensity of light can fall dramatically in a peripheral area along the direction corresponding to the θ// direction to make it no longer possible to reduce the beam diameter. In view of this fact, it is there an ordinary practice that the beam emitted from the semiconductor laser is subjected to an magnification conversion of about 1.4 to 3.0 times to minimize the directional variance in the distribution of intensity of light. For example, since the astigmatism that can be generated in an optical disc device due to the displacement Δ of the light emitting spot from the collimator lens in the forward direction is proportional to Δ×β2, where β is the magnification of conversion, a value slightly smaller than the aspect ratio is normally selected for the magnification of conversion in order to suppress the latter.
Conventionally, an anamorphic prism 101 that is used in an optical disc device is normally prepared by bonding a first prism 102 and a second prism 103 made of respective vitreous materials that different from each other. Prisms of two different vitreous materials are bonded together to produce an anamorphic prism 101 in order to provide the latter with the effect of conversion of magnification and that of “decolorization” and improve the efficiency of manufacturing prisms. The term “decolorization” is used to refer to the effect of preventing the direction of the light beam emitted from the prism from significantly being changed if the wavelength of the light beam striking the prism is shifted from the designed value. The “decolorization” effect is particularly important for the optical disc device of the optical recording modulation type. While the optical disc device of this type is adapted to record signals on a disc-shaped medium by changing the output power of the laser, the laser wavelength can fluctuate at the very start and the very end of its operation. If the anamorphic prism is not provided with the “decolorization” effect, the angle of the light beam coming out from the anamorphic prism can change significantly so that the beam spot formed on the disc by the objective lens can be remarkably shifted from the proper position. Then, the jitter can be increased when the anamorphic prism is used in the direction of scanning density of the optical disc, whereas the problem of a detracked light spot arises for signal recording/reproduction when the anamorphic prism is used in the radial direction of the optical disc.
Conventionally, a technique of combining a vitreous material of the crown glass type having a large Abbe's number whose refractive index can hardly be affected to change as a function of the wavelength of light and a vitreous material of the flint glass type having a small Abbe's number whose refractive index can be greatly affected to change as a function of the wavelength of light is used for realizing the “decolorization” effect for an anamorphic prism as for a lense. For example, an anamorphic prism 101 as shown in FIG. 1 is designed to show a magnification of conversion of 1.9 to incident light with a wavelength of 660 nm. However, such an anamorphic prism can greatly restrict the freedom of designing the configuration of the optical components of the optical head because the direction of the light beam striking the prism and that of the light beam leaving the prism are remarkably differentiated by an angle of 24.63.
FIG. 2 schematically illustrates the arrangement of a known optical head comprising such an anamorphic prism and that of the drive section of a known optical disc device realized by using such an optical head. FIG. 3 is a schematic lateral view of a principal portion of the optical disc device of FIG. 2 as viewed in the direction of arrow D in FIG. 2.
Normally, for forming an optical head, the direction of arrangement of an anamorphic prism is determined according to the divergence angle of the semiconductor laser.
The optical path of the optical head of FIG. 2 will be briefly described here. The laser beam emitted from the semiconductor laser 105 is collimated by collimator lens 106 for the forward direction and then strikes the anamorphic prism 101. The laser beam entering the anamorphic prism 101 is expanded by 1.9 times in terms of a cross section of the laser beam in the direction corresponding to that of θ// to correct the unevenness of the intensity distribution of the laser beam. The laser beam whose intensity distribution is corrected by the anamorphic prism 101 is then emitted from the latter to enter grating 107. The laser beam is then divided by the grating 107 into a principal beam to be used for the purpose of tracking error detection and a plurality of auxiliary beams before being transmitted through the polarization beam splitter plane of polarization beam splitter prism 108. The polarization beam splitter plane transmits p-polarized light and reflects s-polarized light. The leaser beam transmitted through the polarization beam splitter plane is then made to enter ¼ wave plate 109 to become circularly polarized light and then its direction is bent by 90° by a bending mirror 110, which is arranged to make the optical head 104 thin, before the laser beam strikes the objective lens 111. The laser beam that enters the objective lens 111 is then focussed onto the signal recording surface of the optical disc 116 to recording signals on or reproduce signals from the optical disc. The laser beam reflected by and returning from the optical disc 116 is collimated by the objective lens 111 and then its direction is bent by 90° by the bending mirror 110 so that the laser beam strikes the ¼ wave plate 109, which shifts the direction of polarization of the laser beam by 90° relative to that of the forwardly proceeding laser beam. Thus, the returning laser beam whose direction of polarization is shifted by 90° relative to that of the forwardly proceeding laser beam is reflected by the polarization beam splitter plane of the polarization beam splitter prism 108 as S-polarized light and totally reflected by the total reflection plane before entering collimator lens 112 for the backward direction. The returning laser beam that enters the collimator lens 112 for the backward direction is converted into a convergent light beam and then provided with astigmatism by multi-lens 113 for the purpose of focus error signal detection before it is received by a photodetector. The operation of reproducing information and the light spot on the optical disc are controlled on the basis of the optical signal of the returning light beam received by the photodetector.
The known optical device realized by using the known optical head 104 that has the above described configuration can be down-sized because the radial dimension of the optical disc 116 can be reduced and hence the drive can be down-sized.
If θ//=10°, θ⊥=25°, an magnification of conversion β of 1.9 times and an effective NA of the collimator of 0.17 for the forward direction are selected for the distribution of intensity of light on the plane of the pupil of the objective lens and the intensity of light at the center of the pupil of the objective lens is 1, the intensity of light is 0.66 at the outer edge in the direction of scanning density of the tracks and 0.48 at the outer edge in the radial direction of the optical disc. Thus, the intensity of light is less reduced in the direction of scanning density of the tracks.
However, in some optical disc devices, the reduction in the intensity of light is made less in the radial direction of the optical disc than in the direction of scanning density of the tracks. For such an optical disc device to comprise the components of FIG. 2, it has to take one of the three different arrangements as described below. However, the three different arrangements are accompanied by the respective problems as pointed out below.
FIG. 4 shows an arrangement of optical disc device where the semiconductor laser 105 and the anamorphic prism 101 are rotated by 90° relative to each other by taking the relationship of the two intensities of light. FIG. 5 is a schematic lateral view of a principal portion of the arrangement of FIG. 4 as viewed in the direction of arrow E. With this arrangement, since light entering the polarization beam splitter prism 108 is S-polarized, a half wave plate 115 is inserted to change to produce P-polarized light. However, with this arrangement, since the anamorphic prism 101 inclines the optical axis to 24.63°, the optical head 104 has a large height to by turn make the optical device large and, additionally, it is difficult to maintain the desired accuracy level for machining the base on which the components are arranged.
FIG. 6 shows another arrangement of optical disc where the anamorphic prism 101 is replaced by an anamorphic prism 117 that is of the type realized without bonding two prisms due to avoid the inclination of the optical axis on the optical path. FIG. 7 is a schematic lateral view of a principal portion of the arrangement of FIG. 6 as viewed in the direction of arrow F. FIG. 8 is an enlarged view of the anamorphic prism 117. With this arrangement of using the anamorphic prism 117, while both light entering the anamorphic prism 117 and light exiting the anamorphic prism 117 can be made parallel to each other, the component members of the prism including a first prism 118, a second prism 119 and a holder member 120 have to be bonded individually and accurately so that the operation of preparing such an anamorphic prism 117 is a time consuming one and hence the efficiency of manufacturing such anamorphic prisms 117 is inevitably low.
FIG. 9 shows an arrangement of optical disc device where relative angle of the semiconductor laser 105 and the anamorphic prism 101 is held unchanged but all the remaining components are turned by 90°. FIG. 10 is a schematic lateral view of a principal portion of the arrangement of FIG. 9 as viewed in the direction of arrow G. With this arrangement, due to the change in the positional relationship of the optical disc 116 relative to the tracks, both the grating 107 and the photodetector 114 are turned by 90°. While this arrangement does not entail the problem of the height of the optical head 104 and the machining accuracy of the base nor the problem of a low efficiency of manufacturing anamorphic prisms 101, the optical disc is made to show a large radial dimension to make the drive very large so that it is difficult to down-size the optical disc device.